Hot melt adhesives are melted by heating and coated on adherends which are to be bonded, the adherends are then brought into contact with each other, with the melted hot melt adhesive being interposed therebetween, and held in this state, such that the adherends are bonded together. Such hot melt adhesives are used in a variety of fields, for example, in the field of packaging articles such as cardboards and small boxes, the field of sanitary materials such as disposable diapers and sanitary supplies, binding field, plywood field, woodworking field, automotive filed, field of home appliances, and field of household articles.
As mentioned hereinabove, hot melt adhesives are coated after being melted by heating. Therefore, no solvent is required therefor. For this reason, hot melt adhesives have been advantageously used, for example, in the field of sanitary materials as adhesives which are highly safe to the human body. More specifically, hot melt adhesives have been widely used for fixing and assembling constituent members in disposable sanitary materials such as disposable diapers and sanitary supplies.
When hot melt adhesives are used in the field of sanitary materials, they often come into contact with moisture, such as body fluids, due to the nature of application thereof. It is well known that in some hot melt adhesives, the adhesiveness is degraded in contact with moisture, that is, the adhesiveness is low in a wet state. Where such hot melt adhesives are used in the field where the probability of contact with moisture is high, the adhesion between the constituent materials, for example, when the products are used, cannot be maintained, and the products themselves can collapse. For this reason, it is desirable that the adhesiveness of hot melt adhesive be maintained in a wet state thereof.
For example, Patent Literature 1 to 3 disclose adhesives that have improved adhesiveness in a wet state.
Patent Literature 1 discloses an adhesive composition which includes a thermoplastic block copolymer A which is a copolymer of a vinyl aromatic hydrocarbon polymer block and an ethylene-butylene copolymer block, or a mixture of two or more such copolymers, and has at least one carboxyl group in a molecule or has an acid value of 1 or more, and a thermoplastic block copolymer B which is a copolymer of the vinyl aromatic hydrocarbon polymer block and an isoprene polymer, or a mixture of two or more such copolymers.
According to Patent Literature 1, the obtained adhesive composition has improved adhesiveness, in particular, in a wet state, with respect to fiber products such as cellulose-based products, nonwoven fabrics, and paper products, and olefin resin sheets.
Further, Patent Literature 2 discloses a hot melt adhesive including an acid-modified thermoplastic block copolymer, a tackifier resin, a plasticizing oil, and a wax.
According to Patent Literature 2, the obtained hot melt adhesive has excellent adhesiveness with respect to polyolefin resin adherends and cellulose adherends, and exhibits excellent adhesiveness within a range from immediately after bonding to after a long-term storage in both the dry state and wet state of the adherends.
Further, Patent Literature 3 discloses a hot melt adhesive including a thermoplastic block copolymer which is a copolymer of a vinyl aromatic hydrocarbon and a conjugated diene compound, and a wax modified by a carboxylic acid and/or a carboxylic acid anhydride.
According to Patent Literature 3, the obtained hot melt adhesive has improved wet adhesiveness.
In bonding methods using a hot melt adhesive, the hot melt adhesive is melted by heating. The molten hot melt adhesive is then discharged by using compressed air towards an adherend from a nozzle of a coater disposed at a certain distance from the adherend. As a result, the molten hot melt adhesive is coated in a linear shape on the adherend. Another adherend is then brought into contact with the coated hot melt adhesive, and the adherends are allowed to stay to cool and solidify the hot melt adhesive. The solidified hot melt adhesive bonds together the adherends. Further, when the hot melt adhesive is discharged from the nozzle of the coating apparatus, the extrusion direction of the hot melt adhesive is deflected, so as to coat the hot melt adhesive in a specific shape, for example, spirally, to the adherend. Such coating shape of the hot melt adhesive can be selected, as appropriate, according to the required adhesive strength or the like.
Meanwhile, in order to increase the production efficiency of products manufactured by using hot melt adhesives, it is necessary that sufficient adhesiveness between the adherends be ensured at a high rate with a small amount of a hot melt adhesive. More specifically, it is necessary that sufficient adhesiveness between the adherends be ensured even when the nozzle diameter of the coating apparatus is reduced, the flow velocity of the molten hot melt adhesive is increased, and the amount of the coated hot melt adhesive is reduced.
However, where the nozzle diameter of the coating apparatus is reduced and the flow velocity of the molten hot melt adhesive is increased, the flow of the hot melt adhesive inside the nozzle becomes unstable and uniform coating tends to be difficult to perform. In particular, where the uniformity of viscoelasticity of the hot melt adhesive decreases, the pressure or flow velocity inside the nozzle changes abruptly and the hot melt adhesive cannot be sufficiently deflected or the hot melt adhesive is scattered, thereby causing blockage, and the coating shape of the hot melt adhesive can change abruptly. Since the deterioration of coating ability, such as the abrupt variation of the coating shape, can also degraded the adhesiveness. Therefore, good coating ability needs to be ensured not only at the start of coating, but also over a long period of time. Where a hot melt adhesive is coated over a long period of time, the melted hot melt adhesive can be partially retained for a long time inside the coater. It is required that good uniformity of the hot melt adhesive be also maintained in such cases. For this reason, in order to maintain good coating ability over a long period of time, it is necessary to ensure long-term thermal stability of the hot melt adhesive, such that good uniformity is maintained, without the occurrence of accumulation of unmelted material or blockage of flow paths inside the coater.